In P2P systems, each machine connected to the telecommunications network can behave both as a client and as a server. First the digital data received by a machine as a client machine can then for example be served by this machine to other machines in the telecommunications network. Because the shared digital data are kept on the client machines, P2P systems considerably reduce the infrastructure costs compared with centralized systems of the client/server type. This is because P2P systems allow a distribution of the requirements in terms of storage capacity and bandwidth of the network between all the machines connected to the telecommunications network.
A particular difficulty with P2P systems lies in their topology, which is very unstable. This is because, in these P2P systems, the machines can be connected or disconnected at any time. When a machine is disconnected, the data which it is able to supply to a client are then unavailable. The client must then await a subsequent connection of the disconnected machine in order to have access to the digital data requested. In addition, the IP addresses of the machines are unpredictable and liable to be different at each connection.
The accessibility and availability of the data therefore constitute in P2P systems particularly tricky problems for which it would be desirable to offer solutions.
The replication of the data in several servers is a known technique which is used for improving the accessibility and availability of the data in a network.
There is known, through the document WO-02/095605 (IBM), a system for the automatic deployment/redeployment of web services between various sites in a telecommunications network.
In this deployment system, the conditions of use of the web services are monitored and analyzed by a central authority which decides, when it deems it necessary for the efficacy of the services, to trigger a dynamic deployment of the services at various locations in the telecommunications network. The requests from the clients for the services deployed are then redirected automatically to the new locations of these services.
There is also known, through the document U.S. Pat. No. 6,484,204 (AT&T), a system for redirecting requests and managing replicas of objects in which requests for an object sent to an original server are redirected towards replicas of this object on another machine in the network.
In accordance with this AT&T system, the number of redirections of a request to an object replica in question and a distance representing a communication cost as far as each of the servers of the object replica are calculated by a distributing device. When the distributing device receives a request, it determines a server for processing the request from the information calculated. The request is then transmitted to the server selected or, in a variant, it is the requester who is redirected to the server selected.
According to the AT&T system, each machine storing an object replica is free to destroy it, to make it migrate or to replicate it autonomously. In order to decide on the conduct to be followed, the machine compares the number of requests for the object replica with various threshold values. However, the deployment decision, that is to say the decision to replicate on other machines objects held by original servers, belongs in the AT&T system solely to a central authority which is responsible for best managing the object replicas in the network.
These IBM and AT&T systems in the prior art appear to be more particularly designed for networks of the client/server or at least of the centralized type.
Because of the need for a central authority for the deployment decision, the IBM and AT&T systems described above are ill-adapted to an implementation in P2P network architectures of the non-centralized type such as Gnutella.
Deployment decisions taken at each of the servers lend themselves better to use in the various existing P2P network architectures, whether these be centralized, decentralized or hybrid.
Another drawback in particular of the IBM system lies in the fact that the deployment decision is made a posteriori, that is to say after the data concerned have been transferred at least once by the original server. This aspect is detrimental for machines with a low data transfer capacity since, in the best of cases, that is to say if the deployment decision takes place after a first data transfer, these machines will have to transmit the data at least one more time, to a replication server.
The article “Search and Replication in Unstructured Peer-to-Peer Networks” by Qin Lv et al., in ACM International Conference on Supercomputing, June 2002, describes distributed replication strategies in the context of decentralized and non-structured P2P networks such as Gnutella.
The concept of proactive replication of objects in a P2P network is introduced in this article. In accordance with this proactive replication concept, an object can be replicated on a machine without any request having been made by the latter for the object concerned. In this way the search success rate is improved during a search for this object on the P2P network, whilst also improving the availability of the object.
Two replication strategies are presented for the purpose of improving the search success rates. Namely:                a path replication which consists of replicating the object along the path between the requester and the server, and        a random replication which consists of replicating the same number of objects as for the replication above but in which the objects are placed randomly between the sites examined.        
These replication strategies are optimized in order to improve the object search success rate and do not take into account for example parameters related to the data transfer capacities of the machines.
Replication solutions allowing a lesser demand in terms of data transfer of the machines having low transfer capacities and adaptable to various network architectures, including P2P architectures, would constitute a significant advance in this area of the technique.